1. Field of the Invention
The present invention relates to carbon nanotubes for a fuel cell, a nanocomposite including the same, methods of manufacturing the carbon nanotubes and the nanocomposite, and a fuel cell, and more particularly, to carbon nanotubes for a fuel cell to which highly-dispersed nanoparticles of a metallic catalyst are adhered, a nanocomposite including the same, and a fuel cell including the nanocomposite.
2. Description of the Related Art
Recently, with growing concerns about the environmental effects of fossil fuels and the eventual exhaustion of available energy resources, the design and fabrication of fuel cells have received a lot of attention for potential applications such as automobiles and consumer electronics devices.
Fuel cells are new energy conversion devices that transform energy stored in a fuel into electricity through electrochemical reactions of fuel and oxidative gas.
They can be classified into solid oxide electrolyte fuel cells using solid oxide electrolyte, which can be operated at 1000° C.; molten carbonate salt fuel cells, which can be operated at 500-700° C.; phosphoric acid electrolyte fuel cells, which can be operated at about 200° C.; and alkaline electrolyte fuel cells and solid polymer electrolyte fuel cells, which can be operated at an ambient temperature or at a temperature of about 110° C. or less.
Examples of solid electrolyte fuel cells include proton-exchange membrane fuel cells (PEMFCs) utilizing hydrogen gas as a fuel source, direct methanol fuel cells (DMFCs) which generate power using liquid methanol solution directly applied to an anode as a fuel source, and the like. Polymer electrolyte fuel cells are clean energy sources, can replace fossil fuels, and have high power density and high-energy conversion efficiency. In addition, polymer electrolyte fuel cells can operate at an ambient temperature, and can be miniaturized and sealed. These characteristics make polymer electrolyte fuel cells a desirable choice for pollution-free vehicles, power generating systems for home use, portable telecommunications equipment, military equipment, medical equipment, space technology equipment, and the like.
PEMFCs produce a direct current through the electrochemical reaction of hydrogen and oxygen, and contain a proton-exchange membrane interposed between an anode and a cathode.
The proton-exchange membrane is formed of a solid polymer material with good proton conducting properties and minimal cross-over of unreacted gas to the cathode part, such as Nafion. The anode and the cathode include backing layers for supplying reaction gases or liquid, and a catalyst for the oxidation/reduction of reaction gases.
As hydrogen reaction gas is supplied to the PEMFC, hydrogen molecules are decomposed into protons and electrons through oxidation reaction in the anode. These protons permeate across the proton-exchange membrane to the cathode.
Meanwhile, oxygen is supplied to the cathode, and accept electrons forming oxygen ions. These oxygen ions then combine with the protons from the anode to produce water.
A gas diffusion layer (GDL) in the PEMFC is included in the anode and the cathode. The catalyst layer promoting the fuel cell chemical reactions is formed on the anode and cathode backing layers. The anode and cathode backing layers can be formed of carbon cloth or carbon paper.
The direct methanol fuel cells (DMFCs) have a similar assembly structure to the PEMFC described above, but use liquid methanol solution instead of hydrogen as a fuel source. As methanol solution is supplied to the anode, an oxidation reaction occurs in the presence of a catalyst to generate protons, electrons, and carbon dioxide. Although DMFCs exhibit somewhat lower energy efficiency than PEMFCs, the use of a liquid fuel in DMFCs makes their application to portable electronic devices easier.
In order to increase power density and voltage by increasing energy density of a fuel cell, research has been conducted into electrodes, fuels, and electrolyte membranes. In particular, many attempts have been made to increase catalyst activity in electrodes. Typically, the catalyst used in PEMFCs and DMFCs is Pt, Pd, Rh, Ru, or an alloy of Pt and other metals, and a smaller amount of the metallic catalyst is required to decrease manufacturing costs.
The catalyst amount can be decreased while the performance of a fuel cell is maintained or increased. In such a method, a conductive carbonaceous material having a large specific surface area is used as a support and nanoparticles of Pt or the like are dispersed, thereby increasing a specific surface area of the metallic catalyst.
Conventionally, a catalyst such as Pt has been applied as a paste, uniformly onto a porous carbon backing substrate.
However, the dispersion of the catalyst in the backing substrate is not uniform, and the surface area and electrical conductivity of the carbon backing substrate are not large enough. As carbon nanotubes have many attractive physical properties such as good electrical conductivity, superior mechanical strength, high aspect ratio, and large surface-to-volume ratio, various attempts have been made to utilize carbon nanotubes as the fuel cell electrodes (see articles by Li, et al., Journal of Physical Chemistry, Vol. 107, page 6292 (2003); and by Wang, et al., Nano Lett. Vol. 4, page 345 (2004).)
U.S. Pat. No. 6,589,682 discloses the use of carbon nanotubes as a nanoscale gas diffusion layer in the membrane electrode assembly of a fuel cell in order to improve the gas conversion efficiency, electrical conduction, and enhance resistance to mechanical crushing.
The use of high specific surface area of the nanotubes for enhanced catalyst availability is, however, not discussed in this patent.
US Patent Application No. 2004/0018416A discloses a chemical vapor deposition (CVD) method for growing carbon nanotubes on carbon paper for a fuel cell.
However, the use of fuel cell catalyst particles, such as Pt or Ru, and carbon nanotube nucleating catalyst particles is not clearly explained in the application.
In addition, the nanotubes disclosed in the 0018416A patent application are randomly oriented nanotubes, not directionally aligned nanotubes. Random oriented nanotubes tend to tangle and contact each other, thus reducing the available surface area of the exposed carbon nanotubes, as compared to directionally aligned nanotubes which are always separated from each other and fully expose all of the circumferential surface area of each nanotube. Furthermore, the fabrication technique employed in the application is likely to result in a relatively low density of catalyst particles on the nanotube circumferential surface.
Therefore, these is a need to develop a highly gas-reacting efficient fuel cell structure, and hence a need for improved and advanced electrode materials with very large surface area covered with very small, high-density catalyst particles.